The present invention relates to a method for diffusion bonding metals, particularly to compression diffusion bonding of superplastic alloys in superplastic forming (SPF) operations.
The superplastic forming (SPF) and diffusion bonding (DB) process is well documented and has been in use in the aerospace industry for many years. Many kinds of SPF/DB panels are made using a variety of numbers, sizes or thickness of sheets; welding techniques; bonding techniques; stopoff configurations; or other variables. The present invention relates to an improvement in compression diffusion bonding to manufacture integrally stiffened SPF/DB parts. Conventional compression diffusion bonding uses the force reacted between opposed die surfaces of the forming press at elevated temperature to achieve intimate contact and pressure between two or more mating sheets of metal, especially titanium or its SPF alloys. Conventional SPF/DB processes are described, for example, in U.S. Pat. Nos. 5,141,146; 5,115,963; 5,055,143; 4,304,821; 3,924,793; 3,927,817; 4,530,197; 5,330,093; 4,882,823; and 4,292,375, which I incorporate by reference.
Currently compression diffusion bonding requires expensive metal dies, which also require lead flow times to fabricate. The SPF/DB process, for example, for Ti-6Al-4V alloy requires temperatures of about 1650-1750xc2x0 F. with 300 psi differential gas pressure applied using weld-quality argon gas for 3 hours or more under continuous gas purge with intimate contact between clean, debris-free surfaces that have a fine stable grain size.
The state-of-the-art method for compression diffusion bonding uses precision machined, matching, Corrosion Resistant Steel (CRES) plates or dies having an interfacial grid pattern. The SPF titanium material is sandwiched between the dies and is compressed locally at the grid to create a diffusion bond. Forming the dies with the grid pattern is a slow and expensive process because of the limitations on machining that CRES alloys impose. This conventional method is too expensive to justify its use for prototyping, low rate production, or limited quantity production, because making the dies is out of the question. Therefore, designers have avoided SPF/DB designs because they have not been able to verify their integrity at reasonable costs.
Even when implemented, the conventional CRES process is also plagued by the rapid distortion of the die surfaces because of the extended exposure to both high temperatures (1600-1750xc2x0 F.) and large loads required to accomplish diffusion bonding. The opposed surfaces die creep out of shape. Once the die surfaces no longer mate together closely, the compressive forces between the titanium sheets are lost and diffusion bonding no longer occurs. This deterioration of the dies is a particular nuisance because localized disbonding will cause rupturing of the core pack during its intended use, especially in aerospace applications where the part will be subjected to close to its design limits in an effort to reduce overall vehicle weight. In the complete parts the diffusion bonds are often sealed within the part and cannot be inspected. Disbonding poses a potentially catastrophic failure, and inspection is difficult if not impossible. Designers shy away from relying on SPF/DB designs where bond integrity cannot be assured.
While the inability to assure quality diffusion bonds has severely limited the adoption of SPF/DB parts in aerospace applications, making the parts by alternate processes, however, is expensive. Alternatives usually require complicated machining of individual detail parts and their assembly into the completed subassembly. Considerable labor and inspection is required and part variability produces variation in the final subassembly. Fasteners at significant assembly cost. Modern aerospace manufacturing needs the cost savings associated with SPF/DB manufacture, provided that the integrity of the diffusion bonds can be guaranteed.
Another problem designers face with conventional diffusion bonding processes is that there is not a simple way to laser weld or resistance weld complex shapes of core configurations into the inner core sheets of SPF/DB panels. With resistance welding, the designer is limited primarily to straight lines of welded cores. In some instances, gentle arcs can make an extremely competent welder, but the parts are subject to high rejection rates. The laser welding process is better in this regard, but the welding heat distortion caused by the laser can cause severe bending and bowing of the mating sheets leaving areas where intimate contact between the sheets is lost.
The industry needs a reliable compression DB method that enables the rapid fabrication of SPF/DB panels using inexpensive tooling. It also needs a method that eliminates the problems of localized disbanding by assuring intimate contact between sheets throughout the DB cycle. The inexpensive fabrication tools of the present invention allow the construction of numerous sets of tools at only a fraction of the cost of conventional SPF/DB tools. Therefore, a supply of compression DB backup tools can be fabricated and kept in reserve in case those in use are damaged or become distorted. Reserve tools facilitate production runs and guarantee timely delivery. SPF/DB manufacture promises reduced total part count and reduced airplane cost.
Companies that have tried to use the conventional compression DB method to produce DB parts have had difficulties in keeping the die surfaces closely matched. In the case of F-15E parts, the eventual deterioration and distortion of its SPF/DB dies led McDonnell Douglas to try tack welded shims in the areas where disbanding was occurring. The shims increased the stack up thickness of material, thereby theoretically applying more force in those areas where the die had been bent. Unfortunately, the shims simply aggravated the bending of the die, caused even greater misalignment and distortion, and produced an even greater number of disbonds on the parts.
As previously mentioned, a primary problem with using patterned CRES dies are their expense. The dies are typically custom cast close to their net shape from CRES material, such as ESCO 49C, IN100, HN, or 22-4-9 CRES. The die surfaces are then precision ground and machined to obtain a close match, especially where the part will have compression diffusion bonds. The dies are 6-24 inches thick and generally 2 ftxc3x972 ft to 6 ftxc3x9712 ft. CRES alloys are expensive and their machining is timeconsuming. A 3 ftxc3x972 ft CRES die can take up to nine months to construct. The die design must be numerically defined to permit NC machining of the surfaces. Casting can take up to 16 weeks. Machining, handworking, inspecting, and assembly can, then, take another 20 weeks. If rework of the die is needed (welding and machining), the lead time and expense becomes extraordinary. Even a small die design change can take months to accomplish because of the necessary reworking of the dies.
Accordingly, the current method to fabricate compression DB parts for both experimental and production applications is inadequate.
To achieve compression DB of titanium sheets, a tooling setup squeezes the sheets together in the-areas where diffusion bonds are needed. The tooling required for bonding two sheets of titanium consists, in a preferred embodiment illustrative of the invention, of two thick blocks of stainless steel and one laser cut sheet (i.e. a template) of approximately 0.150 inch thick CRES stainless steel (304). The CRES sheet is laser cut with the desired grid pattern corresponding to the location of the bonds in the product. The two sheets of titanium (the production part sheetstock) are cleaned, welded together around their periphery to form a pack, and configured to have gas tubes fusion welded to allow argon gas to enter the pack. The pack is then sandwiched between the stainless steel plates and CRES template between the platens of the SPF press. The entire stack-up is heated. Compression force is applied using the hydraulic ram of the press. Diffusion bonds form in a grid pattern corresponding to the pattern of the CRES template. Gas pressure inflates the pack during the bonding cycle to avoid bonding of areas within the pack where bonding is not desired and keeps them from touching. The template allows significantly higher pressures to be applied to the bond line which reduces the time required for bonding and improves the quality of the bond. Parts built using a single CRES template differ from those that are made using two templates. A single CRES template provides parts having one flat surface and one surface that is formed. Parts made using two CRES templates (one on either side of the pack) will have two pillowed surfaces on the top and the bottom of the part.
The method produces cores for multisheet, complicated DB parts, and is especially designed for the fabrication of cores for 4-sheet DB packs.
Two sheet packs can be compression diffusion bonded in one hour or less. Therefore, this process may be more economical than the methods for making resistance or laser welded cores that are generally in use today. The method of the present invention avoids the 3 hour bonding cycle that is used by the conventional DB method, because of the higher compression forces.
The CRES template can be cut at virtually any draft angle, which allows the part to be diffusion bonded into a grid shape of almost any height and angle. A complicated part can be compression diffusion bonded and formed to the needed shape in one cycle.